The present invention relates to an On-Screen-Display (hereafter abbreviated as OSD) circuit for displaying additional information such as a channel number, present time or the like on a video display screen of a television receiver, for example, and particularly to an OSD circuit having also a function for controlling background color of the video display screen.
Recently, in many audiovisual units, a background color, blue-back color in general, is displayed when no video signal is supplied, that is, when a television receiver is tuned to an invalid channel or a video player is reproducing a non-recorded part of a video tape. Generally, the background color is prepared by the OSD circuit.
FIG. 4 is a block diagram illustrating basic configuration of the OSD circuit, wherein an OSD generation section 1/10 generates OSD signals having RGB (Red, Green and Blue) components for displaying characters and/or symbols indicated by a microcomputer 5, and the OSD signals are mixed with video signals in a mixer 6 to be supplied to a display device (not shown in the drawings).
An apparatus for displaying the blue-back color to be applied in the OSD circuit of FIG. 4 is disclosed in a Japanese utility model registration application laid open as a Provisional Publication No. 84692/'91.
FIG. 5 is a circuit diagram illustrating the prior apparatus provided between the OSD generation section 10 and the mixer 6, comprising a switching circuit 7 and the diodes D1 and D2. The OSD generation section 10 generates RGB signals for displaying the OSD characters and a blanking signal L for making blank areas where the OSD characters are displayed to be supplied to the mixer 6. The switching circuit 7 cuts the R and G signals and sets the B signal at a background color level VB when the blanking signal L is inactive. The diodes D1 and D2 are used for maintaining LOW level of the blanking signal L at the background color level VB.
The blanking signal L is made active when the RGB signals generated by the OSD generation section 10 are to be supplied to the display device. So, when the background color level VB remains inactive, the RGB signals generated by the OSD generation section 10 or the video signals are supplied to the display device according to logic of the blanking signal L. When the background color level VB is active, a blue-back signal BB and a background blanking signal LB both having the background color level VB are supplied to the mixer 6 during the blanking signal L generated by the OSD generation circuit 10 is inactive, and the RGB signals generated by the OSD generation section 10 are displayed on a blue-back screen.
However, in the prior art of FIG. 5, it is needed to assemble many parts, such as the switching circuit 7 and the diodes D1 and D2, besides the OSD generation section 10, as well as to provide an additional circuit for generating the background color level VB. Furthermore, there is a problem in the prior apparatus that when the blue-back signal BB is generated irrespective of the horizontal and the vertical blanking intervals, synchronous signals in the output of the mixer 6 are destroyed, making the outputs of the mixer 6 inconvenient to be reused, to be recorded by another video player, for example.
Therefore, an OSD generation section 10 including a character generator which generates directly the blue-back color signal, when controlled by the microcomputer 5, is used in recent OSD circuits.
FIG. 6 is a schematic diagram illustrating a configuration of this type of the OSD generation section 10, comprising a command processor 2 and a character generator 3.
The command processor 2 generates a control signal CS and control data CD according to control commands CM supplied from the microcomputer 5 of FIG. 4. The control data CD include character data for indicating characters and/or symbols and their color to be displayed and position data for indicating where they are to be displayed. Operating in synchronous with vertical and horizontal synchronous signals V and H (at LOW level when active), the character generator 3 generates the RGB signals corresponding to the control data CD together with the blanking signal L, or the blue-back signal BB and the background blanking signal LB, according to the control signal CS and the control data CD.
FIG. 7 is a block diagram illustrating a conventional example of the character generator 3 of FIG. 6.
The conventional example of FIG. 7 comprises;
a character ROM 31 for storing font data FD, which are read out accessed with a character address AC, PA1 a display control RAM 32 for storing display control data according to the control signal CS and the control data CD, the display control data being accessed with a display address AD for outputting the character address AC and a font color signal FC, PA1 a display address control circuit 33 for generating the display address AD indicating vertical and horizontal positions of the OSD characters according to the vertical and horizontal synchronous signal V and H, being supplied with the control signal CS and the control data CD, PA1 an output circuit 35 supplied with the font data FD, the font color signal FC, the control signal CS and the control data CD for outputting the RGB signals and the blanking signal L or the blue-back signal BB and the background blanking signal LB, and PA1 a clock generator 34 for generating clock signal CK used in the character generator 3 in synchronous with the horizontal synchronous signals H. PA1 a background color control circuit for outputting the three color signals and the blanking signal generated by the character generator, when a background control signal supplied from a command processor is inactive, said background control signal made active when the video picture is to be replaced with a background color, and outputting the three color signals generated by the character generator during the blanking signal is active and background color signals for displaying the background color during the blanking signal is inactive, together with a background blanking signal for suppressing signals of all of the video picture instead of the blanking signal, when the background control signal is active.
Now, operation of the OSD circuit having the conventional character generator 3 of FIG. 7 is described referring to FIGS. 4, 6 and 7.
The microcomputer 5 outputs the control commands CM concerning information to be displayed, which are decoded by the command processor 2 into the control signal CS and the control data CD. The display control data concerning characters to be displayed and their positions on-screen indicated by the control data CD are stored in corresponding display addresses in the display control RAM 32 in the character generator 3. In the display control RAM 32, display addresses mapped corresponding to an OSD format, 12 lines.times.24 columns=288 characters, for example, are prepared.
In each of display addresses corresponding to where no OSD information is to be displayed, a specific character address AC, (display-off address, is written for inhibiting the OSD outputs.
When green characters "AB" are to be displayed on beginning two columns of the first line, character addresses AC of the character ROM 31, where font data FD of `A` and `B` are prepared, are stored in the first and second display address of the display control RAM 32, respectively, together with color data indicating font color signal FC for outputting G signal, while the display-off address is stored in every of other display addresses. Therefore, when the vertical and horizontal synchronous signals V and H are scanning beginning two columns of the first line, where the green characters "AB" are to be displayed, the output circuit 35 outputs G signal, according to the font data FD read out from the character ROM 31 and the font color signal FC output of the display control RAM 32.
Thus, the green characters "AB" are displayed on-screen of a video picture, when video signals are supplied.
When there is no video signal and the blue-back display is indicated by a control command CM from the microcomputer 5, the command processor 2 controls the output circuit 35 to output the blue-back signal BB when the blanking signal L is at LOW level. However, when the display-off address, having a high priority, is stored in the display addresses of the display control RAM 32, generation of the RGB signals and the blanking signal L is inhibited when screen areas corresponding to the display addresses having the display-off address are scanned, letting invalid video signals be displayed. Therefore, every one of the display-off addresses written in the display control RAM 32 must be replaced with a character address AC indicating a filler character whereof all pixels are OFF, for example.
FIG. 8 is a flowchart illustrating above operation for replacing the display-off addresses with the character address AC of the filler character.
For displaying the blue-back color, the output circuit 35 is indicated to output the blue-back signal BB (at step P1) with a control command CM from the microcomputer 5. Then a display address of the display control RAM 32 is set (at step P2) according to another control command CM, and the display-off address therein is replaced with the character address AC of the filler character (at step P3). Steps P2 and P3 are repeated until all the display-off addresses are found (at step P4) to be replaced with the character address AC of the filler character.
Therefore, a certain amount of data for the control commands CM must be transferred from the microcomputer 5 to the OSD generation circuit 10, occupying output port of the microcomputer 5 exclusively.
As heretofore described, in the prior apparatus of FIG. 5, it is needed to assemble many parts, such as the switching circuit 7 and the diodes D1 and D2, besides the OSD generation section 10, as well as to provide an additional circuit for generating the background color level VB. Furthermore, there is another problem that when the blue-back signal BB is generated irrespective of the horizontal and the vertical blanking intervals, synchronous signals in the output of the mixer 6 are destroyed, making the outputs of the mixer 6 inconvenient to be reused.
In another conventional OSD generation section of FIG. 7, there is a problem of the heavy load of control command transfer for replacing the display-off address with the character address of the filler character.